What Depth Should Foundations Be?

Posted by Tim Foster on Nov 29, 2018 2:39:00 PM

What Depth Should Foundations Be

Foundations are the most critical part of a building, and sometimes the most expensive. Most people will advise on 600mm as the standard depth of foundation. Unfortunately, several factors affect the depth of the foundation. Hence, such a one-fits-all solution might be disastrous.

Rankine’s Formula

This offers the quickest way of determining the minimum depth of a foundation. The equation takes into consideration the soil’s density, its gross bearing capacity and the internal friction of the ground. It requires a few lab tests and standard tables to determine these values. Nonetheless, the formula is only used to determine the minimum, not the actual depth.

It is necessary to point out at that a foundation is either shallow or deep: shallow foundations are less than 3 meters or are no deeper than five times the width.

Load Of The Structure

The purpose of the foundation is to transfer the pressure of the building to the ground. Therefore, the shape, height, and weight of the structure are important factors in determining how deep a foundation should be. A skyscraper, for example, will require a deeper foundation than a maisonette with all elements held constant.

Ground Conditions

Rocky or stony grounds are more stable than compressible soils such as clay. As a result, foundations on clay soils might be twice as deep as ones on rocky soils, even if the structural load is the same. Deeper foundations are also necessary for areas such as landfills, swampy areas and water bodies.

The Foundation Of Adjacent Structures

Proximity to the foundation of adjacent structures is another key determinant of a foundation’s depth. The excavation for the new foundation should not undermine the strength of the existing foundation. The general advice is that the foundations be at least at the same depth to avoid coinciding pressure bulbs. The same principle applies when building a foundation close to drainage lines, sewers and water tables.

Frost Level

Frost action is a significant problem in cold countries where the water in the soil freezes. Therefore, all foundations in such regions should be built below the frost level - the level in the ground above which freezing occurs.

Foundations, in spite of their importance, are not accorded sufficient consideration during the planning stages. Nonetheless, it is evident that it is an integral part of the building design stage. You can avoid costly consequences by hiring a skilled architectural designer to work on your structure’s foundation.

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Topics: Architectural Designer

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